The anodes of the present invention may be used for the electrolysis of sodium, potassium, lithium, cesium and ruthenium chlorides and bromides; for the electrolysis of barium and strontium chlorides and bromides; for the electrolysis of other salts which undergo decomposition under electrolysis conditions; for the electrolysis of HCl solutions, for the electrolysis of water and for other purposes. They may be used in mercury or diaphragm cells and may take other forms than those specifically illustrated. However, for the purposes of illustration, the use and construction of our improved anode for the electrolysis of sodium chloride brine to produce chlorine and sodium amalgam in a flowing mercury cathode cell will be described as one embodiment of our invention.
In the production of chlorine by the electrolysis of a brine solution, such as sodium chloride, in an electrolytic cell having an anode and a cathode, the previous most widely used material for anode construction was graphite because of its resistance to the brine and chlorine and other corrosive conditions encountered in an electrolysis cell and its ability to catalyze chlorine discharge from the anodes. Current is ordinarily supplied to the graphite anodes by a copper bus bar arrangement mounted exteriorly to the cell and suitable copper lead-in conductors carry the current to the graphite anodes. The surface of the anode facing the cathode is the working surface or face. Graphite, however, has the serious drawback of wearing poorly during the electrolytic process. Gap width variations, occasioned by wearing and spalling away of the graphite anode working surface, result in the need for additional current to maintain the requisite voltage to cause flow across the electrolytic gap, and the necessity of frequent adjustment or replacement of the graphite anodes. Moreover, the particles of graphite from the anodes collect in the amalgam or electrolyte and result in poor cell performance and additional expense to remove these impurities.
The disadvantages encountered in the use of graphite anodes have lead to attempts to provide dimensionally stable anodes by the use of metal anode structures resistant to electrolytic cell conditions. However, the use of dimensionally stable anodes has been accompanied by problems. The use of wear resistant metals (such as titanium and tantalum) to form dimensionally stable anodes, i.e., anodes with negligible wear and hence constant stability on the working surface under normal operating conditions, has resulted in the frequent occurrence of passivity when used in the brine solution under electrolysis conditions. Passivity results from the formation of a film on the active surface of the metal anode due to oxidization of same or to the inability to catalyze the formation of chlorine (Cl.sub.2) from the chloride ions (Cl-) found at the anode surface. The film on the surface of the metal causes an increase in the electrical resistance of the anode which, in turn, requires that additional current be supplied to maintain flow of current in the electrolysis gap. Moreover, the selection of metal for use in anodes is severely restricted because of the high corrosive character of electrolysis cell conditions and the conductivity of such metals as titanium and tantalum is lower than the conductivity of copper.
The use of platinized titanium anodes which are formed of a titanium or titanium alloy base whose active or working surface is coated with platinum or other platinum group metal has not provided a satisfactory solution of the problems of dimensionally stable anode constructions. While wear, corrosion and passivation are reduced by the use of platinum plated titanium anodes, the cost is exceedingly high and numerous other disadvantages have been encountered. More particularly, peeling of the platinum face frequently occurs because metallurgical technology has not discovered a method of achieving a suitable, lasting bond between these two metals. Also, short circuiting in the electrolysis gap as occurs, for example, when ripples form in a mercury cathode surface, disintegrates the platinum layer, exposing the titanium or other base metal of the anode.
An additional problem encountered in the production of chlorine by the electrolysis of a brine solution with graphite anodes is the difficulty of obtaining and maintaining a uniform electrode gap having uniform voltage over the gap between the anode and the cathode. When graphite anodes are used the wear is ununiform, being greater at the hot end of the cell, a non-uniform gap width results, and the electrolytic process is inefficiently performed when the potential difference across the gap between the anode and cathode is not constant. Using a dimensionally stable anode assures that the gap dimension remains constant over the working life of the anode and definitely improves the cell performance.
The present invention overcomes the heretofore stated problems in the prior art of electrolytic production of chlorine and has as its primary object a dimensionally stable anode which effectively resists corrosive attack while resisting wear along the working surface to assure a uniform gap dimension over the entire width of the gap between the anode and cathode. Moreover, the present invention contemplates a cascade current distribution over the anode to achieve uniform electrical potential over the entire working surface of the anode.
One of the objects of our invention is to provide a dimensionally stable anode which will resist corrosion and other conditions within an electrolytic cell and which will insure uniform current distribution to the anode working surface.
Another object of our invention is to provide a dimensionally stable anode having means to protect the current lead-ins (usually of copper) while insuring uniform distribution of current to the anode working surface.
Another object of our invention is to provide a dimensionally stable anode with means for uniform distribution of current to the working face of the anode, which means will not interfere with discharge of gas bubbles from the working face of the anode.
Another object of our invention is to provide an anode structure in which lead-in protector sleeves are detachable from the anode primary conductor bars, so that the anode is convenient to ship and occupies little shipping space.
Another object of our invention is to provide an anode structure with which lead-in protector sleeves of different length may be used for cells of different height.
Another object of our invention is to provide an anode structure in which the electrolytically active valve metal anode face is removable from the conductors, so that it can be removed and recoated without requiring the conductors to be handled in the recoating operation.
Various other objects and advantages of our invention will appear as this description proceeds.